The invention relates to a ceramic flash TV evaporator.
Flash TV evaporators are used for the evaporation coating of monochrome or color picture tubes with aluminum, in order to produce an electrically conductive layer of a few 100 nm in the picture tubes. This layer constitutes the electrode for accelerating the electrons in the finished picture tube.
FIGS. 1 A/B show a conventional flash TV evaporator in its mounting. In the evaporation coating system, the flash TV evaporator (1) is clamped on both ends between two steel plates (8) with the aid of two screws (2). The steel plates (8) are connected to a current source via copper cables (9). To improve contact, there is generally a graphite sheet (5) between steel plate (8) and flash TV evaporator (1). The dimensions of a customary flash TV evaporator are 4.6.110 mm. It has a cavity (6; FIG. 1C) which is rectangular in the cross section of the flash TV evaporator. The cavity generally has a size of 2.4.40 or 60 mm. The width to which it is clamped laterally at each end is about 15 mm. The length of the flash TV evaporator to be heated is normally about 80 mm.
The picture tubes are evaporation-coated batchwise. A piece of aluminum ( less than 100 mg), generally a cylindrical AL body, a so-called pellet, is placed into the cavity of the flash TV evaporator. The picture tube covering which is to be evaporation-coated is positioned over the flash TV evaporator. The space below the picture tube covering is then evacuated, so that the flash TV evaporator with the clamping system is contained in this vacuum (10xe2x88x925 mbar).
By a direct passage of current, the flash TV evaporator is heated in two voltage steps (preheating/evaporation FIG. 7, FIG. 8) and it evaporates the aluminum contained in its cavity in the process. The time required for this process is in the region of 40 sec to 2 min. The current densities involved briefly reach values of up to 103 A/cm2.
This heavy electrical and therefore thermal loading is what limits the service life, i.e. the number of heating cycles, of the flash TV evaporator.
The flash TV evaporator usually cracks in the region of the cavity after 500-900 heating cycles. It becomes broken and needs to be replaced.
DE 19735814 discloses a flash TV evaporator having an improved service life, improved evaporation behavior and improved wetting behavior which can nevertheless be used in conventional evaporator systems. This flash TV evaporator differs from the flash TV evaporators utilized until then by virtue of its triangular cavity. It is illustrated in cross section in FIG. 1 (D).
The object of the present invention is to provide a flash TV evaporator for the evaporation coating of picture tubes with aluminum having further improved properties.
The object is achieved by means of a ceramic flash TV evaporator, containing a cavity whose width tapers in cross section through the evaporator from the upper edge of the evaporator to the lower edge of the evaporator, which is defined by the fact that the transition of the side walls of the evaporator to the underside of the evaporator is elliptically rounded in cross section at least over part of the longitudinal axis of the evaporator.
The flash TV evaporator according to the invention preferably has a cavity which is triangular in cross section through the flash TV evaporator. In this case, the two side faces of the triangular cavity can be straight (FIG. 2) or curved convexly (FIG. 3) or concavely (FIG. 4). The lowest point of the cavity may be designed as a radius (FIG. 5).
In the extreme case, this leads, when the side faces are convexly curved, to the cavity having a semicircular shape.
In the extreme case, this leads, when the side faces are concavely curved, to a narrow linear indentation in the longitudinal direction of the flash TV evaporator as the cavity.
The cavity preferably has a width of 2 mm to 5 mm at the top surface of the evaporator.
The lowest location of the cavity is preferably at least 1 mm and at most 3 mm below the upper edge of the evaporator.
The length of the cavity is preferably between 30 mm and 80 mm.